Plastics are excellent in moldability, strength, water resistance, transparency, and the like, and therefore are widely used as packaging materials. Examples of the plastics to be used as packaging materials include polyolefin resins such as polyethylene and polypropylene; vinyl resins such as polystyrene and polyvinyl chloride; and aromatic polyester resins such as polyethylene terephthalate. However, these plastics have low biodegradability, and therefore, when they are dumped in nature after use, they remain for a long period of time and therefore may spoil the landscape or cause environmental disruption in some cases.
On the other hand, recently, a biodegradable resin, which is biodegraded or hydrolyzed in soil or water, and therefore is useful for the prevention of environmental pollution, has drawn attention, and the practical application thereof has been studied. As such a biodegradable resin, an aliphatic polyester resin, cellulose acetate, modified starch, and the like are known, but as a packaging material, an aliphatic polyester resin having excellent transparency, heat resistance, and strength, particularly polylactic acid is preferred.
However, such an aliphatic polyester resin has insufficient oxygen gas barrier properties, and therefore cannot be used alone as a packaging material for a content which may be oxidatively degraded such as a food or a drug. Therefore, a laminate having a coating layer formed from polyvinyl alcohol having excellent gas barrier properties and also having biodegradability on at least one surface of a polylactic acid film has been proposed (see, for example, Patent Document 1).
Further, a biodegradable laminate which uses a melt-moldable polyvinyl alcohol resin having a 1,2-diol structure in its side chain and is obtained by coextrusion-laminating an aliphatic polyester resin whose melting point is different from that of the polyvinyl alcohol resin by 20° C. or less on both surfaces thereof (see, for example, Patent Document 2).